Nuclear fusion seems to have always been thirty years away, but the seeming interminable quest appears to be edging ever closer with the construction of the ITER reactor in the South of France.
From the moment that Robert d’Escourt Atkinson and Fritz Houtermans collaborated in 1929 to demonstrate George Gamow’s quantum tunnelling theory the chase for nuclear fusion was on. They proved that by fusing light nuclei you could create energy in accordance with Einstein's formula of mass-energy equivalence and that heavy nuclei could be built up by a successive series of fusions.
But the road to harnessing that theory and deliver commercial nuclear fusion energy has been long and tortuous leading many sceptics to predict that it will never be a viable solution to our energy woes.
“Fusion was overhyped and oversold in the 1960's and we have been cursed by that overselling ever since,” David Martin, head of operations and programme at Culham Centre for Fusion Energy (CCFE) explains. “You can almost guarantee that in every meeting of over five people someone will ask the question 'is fusion still 30 years away?'. You can choose to measure in years or some other currency, but if we have significantly more funding then we could solve the problem significantly quicker.
“We could actually build a reactor now, but it would not be economic because whilst the neutrons give up their energy and produce the heat we need to generate steam, they also damage the materials we have available now. The physics of fusion is now well mostly understood and resolved, but what is not resolved is the engineering consequences of generating these neutrons.
“You could build a reactor now with today's materials, but it wouldn't be economic because you would have to build a new reactor or remove and replace to core of the machine within two years. This includes everything inside the plasma chamber; ten billions dollar’s worth of plant.
“The machines we have available at the minute are designed to only run for short bursts. The JET can only sustain plasma for 30 or 40 seconds because the coils get hot. In principal you could keep the plasma running for hours, but your coils and power supplies would basically cook. It is not a physics limitation but a balance of plant limitation.”
The ITER project under construction in the South of France is the next step towards commercialising fusion power and Michel Claessens, head of communications at ITER echoes that sentiment. “In the current global energy situation fusion is very important, although we are still quite a long way before fusion will be a commercial energy source,” he says. “We do not expect commercial to be commercial before 2050.
“The point that 'fusion has always been 30 years away' is one that is often brought to the attention of the media and public. There is some truth in the fact that we have been close to fusion for some time in that we are already producing fusion energy in the tokomak. There are about 20 tokomaks in the world that work very efficiently, but they are more for the scientific community. These tokomaks have produced fusion power so we are there already. The fact is that we have not succeeded in producing more energy than the power that is injected into the machine, so from that point of view we are not yet within the commercial phase. ITER feel it will be different, and it should be different, because we will for the first time we will produce a net balance of energy, we will produce ten times more energy than we inject into the machine.”
Fission or fusion
There is considerable confusion amongst the general public between nuclear fusion (the coalescence of two nuclei) and the current energy derived from nuclear fission (splitting the nucleus). The explanation lies in the size of the nuclei.
Light elements, such as hydrogen and helium, have small nuclei that release lots of energy when they fuse together. Moving to heavier atoms, less energy is released in each fusion event; until, at iron (26 protons and 30 neutrons); no more energy is released by fusion. Any bigger, it takes energy to make fusion happen. Atoms with really huge nuclei, such as uranium and plutonium do the opposite of fusion: they release energy when they break apart. This is nuclear fission, the process which powers current nuclear power plants.
Fission reactions are much more complex than fusion. For example, uranium-235 can break apart a number of different ways, and many of the atoms produced are unstable and radioactive. This is one attractive thing about fusion, the reaction products are not radioactive: Helium is one of the most stable (and, in a balloon, fun) elements known.
The quest began in earnest in the late Seventies with the construction of JET at CCFE, near Oxford. At present it is the world’s largest and most powerful tokamak and the focal point of the European fusion research programme. Designed to study fusion in conditions approaching those needed for a power plant, it is the only device currently operating that can use the deuterium-tritium fuel mix that will be used for commercial fusion power.
Since it began operating in 1983, JET has made major advances in the science and engineering of fusion, increasing confidence in the suitability of the tokamak for future power production.
Milestones at JET have included the world's first controlled release of deuterium-tritium fusion power (1991) and the world record for fusion power (16 megawatts in 1997). In recent years, JET has carried out much important work to assist the design and construction of ITER. After more than 25 years of successful operation, JET is still at the forefront of fusion research and is closely involved in testing plasma physics, systems and materials for ITER.
Over the almost 30 years that Jet has been in operation it has greatly enhanced the understanding of fusion energy and giving the international community the confidence to fund the ITER project. “What we do here is about understanding different things in the fusion process,” Martin continues. “We need to understand how the fuelling system works. You run the tritium through the system and then you have to extract it out of the exhaust and re-use it. It doesn't go up the stack otherwise you would use the world reserves very quickly so you have to reuse it. You need to understand how to run a tokomak in a stable mode, which yields the neutrons that you need to generate the powers that you need. It is understanding how to build the thing and what the right configuration is.”
Also located at CCFE is MAST (Mega Amp Spherical Tokamak), the UK's fusion energy experiment, which along with NSTX - a complementary experiment at Princeton in the USA is one of the world's two leading spherical tokamaks (STs).
Experiments on MAST are important because they test ITER physics in new regimes and they help determine the long-term potential of the ST, which may eventually be suitable as the basis for a power station.
“At MAST, which is a much cheaper device than ITER, we can test outside of ITER some of the power plant scenarios,” Martin explains. “We will continue to do that and are currently undergoing a £30m upgrade that will keep it as a world-leading device for the next ten years or so. There will be continued exploitation of that. We are also changing from being a research-based institute to one that will be one based on technology. There will be a transition over a period of time to looking at the technology around fusion power and not just experimental side of things.”
In JET and ITER, the fusion reaction will be achieved in a tokamak device that uses magnetic fields to contain and control the hot plasma. The fusion between deuterium and tritium (D-T) will produce one helium nuclei, one neutron, and energy.
The helium nucleus carries an electric charge which will respond to the magnetic fields of the tokamak and remain confined within the plasma. However, some 80 per cent of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the tokamak, transferring their energy to the walls as heat.
In ITER, this heat will be dispersed through cooling towers. In the subsequent fusion plant prototype and in future industrial fusion installations, the heat will be used to produce steam and by way of turbines and alternators electricity.
The next step
Currently under construction in the south of France ITER will be a scaled-up version of JET, with linear dimensions twice the size, and ten times the plasma volume. In ITER, scientists will study plasmas, with a major radius of six metres, in conditions similar to those expected in an electricity-generating fusion power plant. It will also test a number of key technologies for fusion power stations, including superconducting magnetic coils, the blankets surrounding the plasma which will breed tritium and absorb the neutrons' energy, and remote maintenance.
Construction began on ITER in 2010 and is not due to be completed until 2018. In total 39 buildings will be built and at the moment just three of these are under construction. “We will have a peak of construction work during 2014 and 2015 where over 3,000 workers will be on the site,” Claessens explains. “At the moment from my office you can see that it relatively calm with around 200 workers on site each day. The site itself is 42 hectares, which is around 50 football pitches, so it is huge. It is one of the biggest work sites in Europe at the moment.”
Design for the reactor is almost finished and the manufacturing of the components is well underway with over 80 per cent of the contracts signed. As with all internationally funded projects this procurement process is complicated by the division of labour and contracts between the funding nations.
“ITER is a prototype,” Claessens says. “We will not produce any electricity and we are basically demonstrating fusion is possible on Earth in a controlled way. We will be showing that we can produce ten times more energy that is injected into the machine. Then we have to demonstrate that there are good prospects for the commercialisation of fusion: we need to show we can manage longer pulses and that we can also produce some tritium within the machine itself.”
The first experiments will begin in 2020, but tritium will not be used until 2027. “Between 2020 and 2027 it will be essentially testing the machine, adapting the parameters and understanding controlling the plasma. We will be produce high energy from 2027.”
Despite the high level of understanding and confidence in the fusion process there remain several key challenges that need to be overcome before we are able to move on to the next stage of the process, a commercial scale demonstration plant. “Firstly, to run a tokomak and produce fusion energy, which we have done a couple of times now, is most definitely the biggest.” Martin says. “JET did this for the first time in 1991 and then again in 1997. It is the understanding of how to run a tokomak in long, stable modes.
“These things, when they are used as power stations, can actually be very boring. When we do experiments we are pushing back the boundaries of our understanding of the science and it is not risky, but interesting. A power station would not be running like that; you would want very boring, very mundane plasmas.”
Another challenge faced by the engineers at ITER is to determine what would be the material inside the reactor for a commercial reactor. “The walls of the reactor will be under very high loads of energy and we do not know which material will resist this,” Claessens says. “There is a specific Europe-Japan collaboration that is starting now on this but the materials will be tested at ITER.”
A final challenge that will need to be overcome is to develop a safe and efficient manual handling system. “Manual handling will be required in an even bigger way in a commercial plant than we currently use it in experiments,” Martin explains. “We have had the success of understanding how to maintain things remotely, especially because of ALARP (as low as reasonably achievable radiation doses to people). With JET you can actually go in if you need to, but we don't because of ALARP. We limit the dosage of people by using robotics.”
Future for fusion
As Claessens explains ITER is still an experiment, it is not a prototype reactor. That step will have to wait until ITER’s successor, not expected until near the mid-century point, but at that time we will hopefully see fusion power connected to the grid for the first time.
“The biggest step forward that ITER will provide will be the long pulse of an hour long and the challenges of providing and maintaining that,” Martin adds. “There is a control system continually tweaking these pulses to keep them stable. You can have various modes like a low mode, high mode or hybrid mode that these devices run in and the understanding of that has grown enormously over the duration of JET's operation.”
Patience is the key
“I absolutely believe we are on the right path,” Martin concludes. “Patience and waiting for all of the pieces to fall into place. This is a huge jigsaw that we are slowly fitting the pieces together but the rewards are such that it makes the wait worthwhile.”